The objective of our project is to understand the mechanism by which a visual pigment molecule both excites and light adapts a photoreceptor. Our approach is to measure the extent to which excitation (light induced conductance change) and adaptation are localized within single photoreceptors. We plan to use local excitation and adaptation as tools for studying substances (notably calcium) presumably involved in the transduction process. BIBLIOGRAPHIC REFERENCES: Fein, A. and J. Lisman. 1975. Localized desensitization of Limulus photoreceptors produced by light or intracellular calcium ion injection. Science 187:1094. Fein, A. and J.S. Charlton. 1975a. Local membrane current in Limulus photoreceptors. Nature 258:250.